The formation of calluses is primarily a result of friction. As the layers of skin are loaded in a shearing fashion, the planes of skin separate. This leads to blistering in the space between layers. With further progression of shear loads, the upper layer or layers of skin can be traumatized to the point where it separates from the deeper layers. This results in painful, raw, exposed dermis. In addition to the pain associated with exposure to these deeper layers, there is a danger of progression of the sore as successive layers are forcefully torn away. Ultimately, this can lead to open sores called ulcers. Ulcers occur when the depth of the wound has advanced through the epidermis, dermis, and into the subcutaneous fat layer. This layer is highly vascular, and susceptible to infection.
Separation of layers of skin that led to this destructive process is a result of mechanical forces. In particular, the skin structure can be traumatized by vertical forces, perpendicular to the skin, or by shear forces, in the same plane as the skin, with shear forces being the primary culprit. It is these excessive shear forces that are the primary mechanical cause of various skin pathologies and a contributing factor to the failure of medical treatment modalities such as skin grafts. For many people excessive shear force is the primary cause of blistering during day-to-day activities and during high impact activities that occur in many sports. An interface that is capable of reducing or eliminating shear forces would greatly reduce the potential for formation of blisters, and reduce the risk of subsequent ulcers and infection. This is particularly a problem in many medical conditions where the patient has reduced sensitivity as a result of disease or medical procedure. These patients may be unaware of the formation of such skin lesions or ulcers until they are quite advanced. In fact, the leading cause of non-traumatic amputation of a leg or foot is infection following ulcer formation in diabetic patients with neuropathy. In the US alone, nearly 60,000 amputations are performed annually due to non-healing ulcers, with an annual cost in excess of $2 billion.
In the medical field, attempts to reduce the shear force have utilized various polymers in the form of dimensional foams or gels. The idea was to have the material compress and rotate so that the shearing forces were taken up within the material and not at the material skin interface.
The problem with this type of apparatus is not only that the amount of reduction in the shear dependent on the property of the material, but it is also dependent on the thickness of the material. The thicker the material, the greater the reduction in shear forces. To provide adequate amounts of shearing between surfaces of the material there must be a nominal dimensional thickness to the foam or gel. As the material gets thinner less motion between surface layers occurs, thereby limiting its usefulness in reducing shear forces. So the limitations of the dimensional polymers to reduce friction is dependent on their thickness and their unique chemical make up. How much side to side motion the top and bottom layers can move is dependent on how far each polymer can give or slide before the combined force overcomes the shear force. When this occurs the skin will slide on top of the foam producing greater shear forces or the polymers will break. This break down is an additional problem with dimensional polymers. Under prolonged shearing force the material eventually fatigues and fails. This results in material compression or more commonly cracks and tears.
Likewise, with athletic equipment, such as socks, the problem of blistering after extended periods of activity is well known. When an athlete endures high physical stress, the magnitude and frequency of the skin rubbing against the inner surface of a sock or other high-impact area, is increased when compared to normal daily activity. Thus, the blistering caused by such shearing forces is a common ailment of many athletes. The ability of a sock to prevent this blistering has been heretofore limited to different materials and weaves, principally for the purpose of providing cushioning. Providing a sock with reduced shear forces is unknown. The same is true of gloves, points of contact with various padding, and other athletic equipment.